I am considering buying canon 10-22mm and wil use it with 6 inch doom(ikelite). Now the question: most people say you need at least 8 inch, but because of transport isseus 8 inch not a option.
So is it worth to use 10-22mm inside 6 inch doom?
I now use sigma 18-50 2.8 EX DC macro and have never notice any problems with sharpness.(6 inch doom)
I know there are other lenses, but that is an other topic.

I would definitaly recommend the 8inch.
I am using an ikelite housing for a canon 20d. I also have the 10-22 mm, and without a diopter on around f5.6 it does suffer from fuzzy (unsharp) corners (Havent tried the diopter yet.). That will be worse with a 6 inch dome.. Your better guess would be a fisheye type lens (sigma 15mm or tokina 10-17mm), they dont seem to suffer from that problem as much.

I shoot the 10-22mm behind a Zen 200mm dome and it performs quite well at f/5.6 or smaller. But to be fair, its my DX lens of choice for pelagics / sharks so corners are not that important when shooting into the blue. If you need detailed corners I very much suspect that a 6" port will not deliver with the Canon glass and as Gerard suggests, you may wish to consider a Tokina, but with a Kenko x1.4 which will give you the reach of the 10-22mm and largely mitigate the barrel distortion of the fisheye. Your 6" port won't be too shabby for CFWA shots with the Tokina / Kenko combo too. Remember, rectilinears need big domes for sharp corners and for definitive advice, please take a look at Alex Mustard's presentation at DEMA -